CURRENT RESEARCH IN AIRCRAFT TIRE DESIGN

AND PERFORFlAMCE

John A. Tanner and John L . McCarty NASA Langley Research Center

S. K. Clark University of Michigan

S UQlMA R Y

A review of the NASA experimental and analytical t i r e research programs which address the various needs identified by landing gear designers and a i r - plane users i s presented in th i s paper. The experimental programs are designed t o increase t i r e tread 1 ifetimes , re la te s t a t i c and dynamic t i r e properties , establish the t i r e hydroplaning spin-up speed, study gear response to t i r e fa i lures , and define t i r e temperature profiles d u r i n g taxi , b rak ing and corner- ing operations. The analytical programs are aimed a t providing insights into the mechanisms of heat generation in roll ing t i r e s and developing the tools necessary t o streamline the t i r e design process and t o aid in the analysis of landing gear problems.

INTRODUCTION

For many years t i r e researchers a t Langley have maintained a close working relationship with the a i r c r a f t landing gear community, namely, the airframe manufacturers and the a i r l i n e operators. This relationship has allowed NASA t o keep abreast of constantly changing t i r e research needs and t o adjust i t s program priori t i es accordi ngly , Recent inputs from various sources have i ndi - cated that research i s needed t o (1) improve t i r e lifetimes bo th in terms of reduced tread wear and greater blowout resistance; ( 2 ) solve such landing gear dynamic problems as shimmy, gear walk, truck pitching, and braking and corner- ing performance in adverse weather; and (3) streamline the t i r e design process. NASA currently has several research programs underway t o address these needs and the purpose of t h i s paper i s t o present a s ta tus report on these ac t iv i t i e s and t o provide some indication as t o the direction of future research e f for t s .

TREAD NEAR

Tire replacement due t o tread wear i s a major safety and economic concern For th i s reason, NASA has since the early 1970's ( re fs . of a i r l i ne operators.

1 t o 3) been involved in a program t o examine the effects of ti 're tread wear attr ibuted to the various ground operations of an airplane. attempts are underway t o develop new elastomeric materials which would provide

In addition,

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improved t i r e tread wear and blowout resistance w i t h o u t degrading traction characterist ics. For the purpose of t h i s program, braking and cornering t e s t s are being conducted on specially-prepared t e s t t i r e s using the instrumented vehicle shown i n f igure 1 . which i s cantilevered from the rear of the truck and supports the t e s t t i r e . For braking t e s t s a t fixed s l i p ra t ios the t e s t t i r e and wheel assembly i s driven t h r o u g h a universal coupling by interchangeable gears, which i n t u r n a r e chain-driven by a d r i v i n g wheel on the truck. merely changing the gears in the drive unit and i n this program the s l i p ra t ios are being varied from 0% t o 50%. i s disconnected and the f ix ture i s rotated t o the desired steering or yaw angle and clamped in place. Yaw angles t o 25-degrees are being evaluated. The braking t e s t s are being conducted on both asphalt and concrete runway surfaces, b u t the cornering t e s t s a re being limited t o a re la t ively smooth asphalt surface.

The main feature of this vehicle i s the t e s t f ix ture

Changing the s l i p r a t io en ta i l s

For cornering t e s t s the universal coupling

A sample of the wear, f r i c t ion , and temperature data obtained during th i s t e s t program i s presented i n figure 2 . type VII, 12-ply r a t i n g a i r c ra f t t i r e s which had been retreaded w i t h four d i f - ferent elastomers whose compositions were a s follows:

These data are for s ize 22 x 5.5,

El as tomer Compos i t i on 100% natural rubber 75% natural rubber and 25% cis-polybutadiene 75% natural rubber and 25% vinyl-polybutadiene 75% natural rubber and 25% trans-polypentenemer

Elastomer A was tested because natural rubber has been considered the elastomer best suited t o meet the t i r e requirements for supersonic transport- type a i r c r a f t . Elastomer B comprises a stock representative of the current state-of-the-art treads for j e t transports, and elastomers C and D a re experimental blends developed specif ical ly for t h i s program. A f i f t h experi- mental elastomer which consists of a tri-blend of natural rubber, cis-poly- butadiene, and vinyl-polybutadiene, i s currently being evaluated. Presented on the l e f t of f igure 2 are plots of the t i r e wear ra te as a function of the s l i p r a t io from the b r a k i n g t e s t s and yaw angle from the cornering t e s t s ; a lower wear ra te indicates a longer t i r e tread l i f e . the 100-percent natural rubber tread (elastomer A ) has the highest wear ra te and the state-of-the-art tread (elastomer B ) has the lowest wear ra te of the elastomers tested to date. Treads fabricated from the two experimental elastomers had similar wear character is t ics b u t neither had a wear resistance as good as the present state-of-the-art elastomer. the experimental treads such as heat buildup, cut growth , and heat blowout resistance could, however, t i p the balance i n favor of one of the experimental elastomers for some applications. I t i s apparent from the figure t h a t ex- tended operations a t high s l i p ra t ios and/or h i g h yaw angles s ignif icant ly shorten tread l i f e for a l l tread materials.

The d a t a indicate t h a t

Other character is t ics of

While experimenting with the composition of the t i r e tread t o improve i t s wear character is t ics i t i s important that the t i r e f r ic t ion capabi l i t ies n o t be compromised. e i ther the braking or cornering modes are faired by a single curve in figure 2 ,

The f r ic t ion measurements obtained from the various t i r e s in

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thereby suggesting tha t the various tread elastomers studied t o date do n o t significantly a f fec t the t i r e f r ic t ion performance.

Also presented i n f igure 2 a re the maximum tread temperatures as obtained

Tread temperatures were observed t o

from an optical pyrometer mounted on the t i r e f ix ture which continuously monitored the tread temperature of a p o i n t on the rotating t i r e approximately 3/8 of a revolution out o f the footprint . be independent of the elastomer and the figure shows tha t this temperature increases with increasing s l i p r a t i o and increasing yaw angle.

STATIC AND ROLLING TIRE BEHAVIOR

NASA research on s t a t i c and rol l ing behavioral character is t ics of the pneumatic a i r c r a f t t i r e i s concerned w i t h studies of gear response to t l r e fa i lures (blowouts); t i r e carcass temperatures d u r i n g various a i r c r a f t ground operations; wet runway friction/hydroplaning; and certain key t i r e mechanical properties. The following paragraphs briefly discuss each of these planned or on-going programs.

Gear Response t o Tire Failures

The need for a study of landing gear response to t i r e fa i lures has become

A tentat ive t e s t matrix has been more c r i t i c a l in recent years due t o the increasing number of t i r e fa i lures experienced by the wide-body airplane f l e e t . outlined for a NASA program which ca l l s for studies of the f r ic t ion forces developed by deflated (blown) t i r e s and by wheels roll ing on rims; an assess- ment of debris trajectory patterns associated with t i r e blowouts; and the response of the s t r u t , antiskid braking system, and the demands placed on the nose gear steering system following a main gear t i r e fa i lure . A planning session has been scheduled with representatives from a i r l i ne , t i r e , and other aviation industries and interested government agencies t o f ina l ize the t e s t matrix and testing will probably commence in mid-1981.

Tire Carcass Temperatures

The generation of heat i n a i r c r a f t t i r e s i s undergoing study to determine the temperature profiles which are necessary to define the strength and fatigue limitations of the t i r e carcass structure. Both experimental and analytical e f for t s are currently underway in this study. The experimental t e s t s are being carried o u t on s i ze 22 x 5.5 a i r c r a f t t i r e s and the t e s t vehicle i s the same instrumented truck shown i n f igure 1. For these t e s t s , the t i r e s are equipped with a number of thermocouples located w i t h i n the t i r e carcass on one side of the t i r e centerline a s i l lus t ra ted i n the schematic of figure 3. photograph of one such t i r e instal led on the t e s t vehicle i s presented i n f igure 4 which also shows the modified hub and s l i p r i n g assembly for trans-

A

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mitting the thermocouple signals t o the on-board recording equipment. erature data a re being acquired while the t i r e i s operated under free-roll ing, light-braking, and yawed-rolling conditions. Since only one side of each t i r e i s instrumented, symmetry i s presumed a b o u t the t i r e centerline for free- roll ing and light-braking conditions. For yawed rol l ing conditions, t e s t s a re r u n a t yaw angles of equal magnitude on e i ther s ide of Oo to account fo r any asymmetrical heating conditions. Typical resu l t s from the free-roll ing t e s t s are presented i n f igure 5 which shows the carcass temperature pro-files of a t e s t t i r e a f t e r travell ing distances of 1500 m , 3000 m, and 4500 m a t a ground speed of 17 knots. The data indicate that the hottest portions of the t i r e carcass are beneath the tread near the shoulder area and along the inner sur- face of the sidewall.

Temp-

The analytical e f fo r t t o model the heat generation mechanisms w i t h i n an a i r c r a f t t i r e i s being conducted a t the University of Michigan under a NASA g r a n t . cross section and t r ea t s the heat generated w i t h i n the t i r e as a function of the s t r a in energy associated w i t h the predicted t i r e flexure.

The model employs an assembly of f i n i t e elements to represent the t i r e

Figure 6 presents a comparison between the experimental data and the resu l t s from a preliminary analysis performed on a free-rolling t i r e . f igure shows the temperature r i s e as a function of time as measured and cal- culated a t two thermocouple positions for two t i r e deflection conditions. thermocouple positions include the inner and outer surface of the t i r e sidewall near the bead, and t he i r positions are denoted on figure 3 by asterisks. The data presented i n f igure 6 show good agreement between the experimental and the calculated temperature r i ses . Current analytical work i s aimed a t refining the s t ra in energy terms i n the model t o provide even closer agreement i n the free-rolling case and t o address the braking and cornering cases.

The

The

Wet Runway FrictionlHydroplaning

For many years the Langley Research Center has been associated with f r ic t ion and hydroplaning research. r e su l t of these early research e f for t s , the c r i t i ca l hydroplaning speed a t which the t i r e begins to spin down when water i s encountered on the runway has been well established. These early t e s t s also indicated that a speed reduction t o a second, lower c r i t i ca l speed i s necessary t o allow the t i r e to spin up again, b u t t h i s c r i t i ca l hydroplaning spin-up speed has not been well documented. establish the speed reduction necessary t o allow t i r e spin-up, and to measure the hydrodynamic pressures within the footprint .

(See re fs . 4 and 5 for examples.) As a

Tests are currently underway on the Langley Landing-Loads Track to

Tire I4echanical Properties

NASA's continuing investigation of t i r e mechanical properties ( re fs . 6 and 7 are typical of e a r l i e r work i n t h i s area) was recently expanded to

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support a program sponsored by the Soc ie ty o f Abtomotive Engineers (SAE) t o measure both the s t a t i c and dynamic p roper t i es of two s izes o f modern a i r c r a f t t i r e s . Data were generated us ing 49 x 17 and 18 x 5.5, t ype V I 1 t i r e s and prov ided some i n s i g h t i n t o t h e r e l a t i o n s h i p of c e r t a i n s t a t i c and dynamic t i r e p roper t i es . An example i s shown i n f i g u r e 7, which presents the l a t e r a l sp r ing r a t e s determined from s t a t i c and dynamic t e s t s performed on a 49 x 17 s i z e t i r e . The key t o r e l a t i n g the s t a t i c and dynamic data l i e s i n the i n t e r p r e t a - t i o n o f s t a t i c l oad -de f lec t i on curves s i m i l a r t o t h e one sketched i n f i g u r e 7. As w i l l be noted, t i r e s t a t i c l oad -de f lec t i on curves generate a subs tan t i a l hys teres is loop. sp r ing r a t e and genera l l y , as i n references 6 and 8, t h i s r a t e was de f ined by the s lope o f the l i n e which connected t h e loop extremes. Unfor tunate ly , these sp r ing ra tes were always lower than those obta ined du r ing dynamic t e s t s . Observations from t h e SAE t e s t program suggested t h a t two sp r ing r a t e s would be more representa t ive o f t i r e response t o s t a t i c loadings. One r a t e would be the slope o f t he l oad ing p o r t i o n o f t he s t a t i c hys te res i s loop and the o the r r a t e would be de f ined by t h e i n i t i a l s lope o f the l oad r e l a x a t i o n curve f o l l o w i n g at ta inment o f the peak s t a t i c load. Both o f these r a t e s and t h a t obta ined from dynamic ( f r e e v i b r a t i o n ) t e s t s a re presented i n f i g u r e 7.. t h a t the two s t a t i c a l l y determined ra tes de f i ne an envelope which would i nc lude a l l the poss ib le sp r ing r a t e s obta ined under dynamic l oad ing cond i t ions , w i t h the lower bound o f the envelope de f ined by the s t a t i c load ing curve and the upper bound de f ined by the i n i t i a l s t a t i c r e l a x a t i o n curve.

It has been customary t o ass ign a s i n g l e va lue t o the t i r e

The f i g u r e suggests

ANALYTICAL T IRE MODEL DEVELOPMENT

NASA i s developing a f a m i l y o f a n a l y t i c a l t i r e models which w i l l be usefu l i n t i r e design and land ing gear ana lys is . NASA, the Col lege o f W i l l i am and Mary, and George Washington Un ive rs i t y . The approach being taken i s t o use f i n i t e elements based on non l inear s h e l l theory (refs. 9 and 10). The s h e l l theory i s l i m i t e d t o smal l s t r a i n s b u t can handle an iso t rop ic , nonhomogeneous, e l a s t i c ma te r ia l c h a r a c t e r i s t i c s ; bending exten- s iona l coupl ing; l a r g e deformations; and moderate r o t a t i o n s . To date two q u a d r i l a t e r a l s h e l l f in i te -e lement models have been developed which fea tu re the use o f a reduced-basis s o l u t i o n a lgo r i t hm and automat ic s e l e c t i o n o f load o r displacement incrementat ion ( re f s . 11 and 12). One model i s a 16-node f i n i t e element based upon the displacement fo rmu la t i on and the second model i s a f i n i t e element based upon a mixed fo rmu la t i on w i t h 9 nodes a long the per iphery where displacements a re the fundamental unknowns and 4 i n t e r n a l nodes where the s t ress r e s u l t a n t s a r e the unknowns. For the mixed fo rmu la t i on the i n t e r n a l s t ress r e s u l t a n t s are d iscont inuous across the in te re lement boundaries and the s t ress parameters and t h e i r path d e r i v a t i v e s a re e l im ina ted on the element l e v e l . The performance o f these two s h e l l t i r e models i s being v e r i f i e d by app ly ing i n f l a - t i o n pressure loads. Typ ica l r e s u l t s f rom these v e r i f i c a t i o n s tud ies a r e presented i n f i g u r e 8 f o r a 10-ply t i r e o f e l l i p t i c a l cross sec t i on mounted on a r i g i d wheel. A schematic o f the model cross sec t i on i s presented on the l e f t o f the f i g u r e . Also presented i n the f i g u r e i s a p l o t o f the pressure load aga ins t t he crown displacement o f t h e t i r e i l l u s t r a t i n g the hardening sp r ing c h a r a c t e r i s t i c o f the t i r e . model geometries a r e shown t o scale and i l l u s t r a t e the 'large deformat ions

This i s a j o i n t venture between

The drawing o f the u n i n f l a t e d and i n f l a t e d t i r e

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associated with this simple loading system.

Future work in t i r e analytical model development will be concentrated i n the major areas of material characterization, modeling techniques, and load determination. In the area of material characterization the next e f fo r t will be aimed a t the extension of the current l inear e l a s t i c material model t o includr the nonlinear e f fec ts of viscoelastic material behavior. T h i s e f fo r t will also include studies of the effects of elevated temperature on the material strength and i t s mechanical properties. Modeling techniques i n the future will include rational approximations t o the comprehensive nonlinear shell theory and solution algorithm refinements tha t reduce computer costs while maintaining solution accuracy. Future loading systems imposed upon the model will be consistent w i t h studies of the tire/runway contact problem, the inclusion of braking and cornering forces, and the investigation of the dynamic effects of roll ing.

The importance of the analytical t i r e model development program can best be expressed in terms of i t s anticipated applications. t i r e models should furnish a means of streamlining new t i r e development and qualification procedures. Furthermore, these analytical tools should be able t o predict t i r e f a i lu re modes during the design phase so t h a t appropriate steps can be taken t o prevent undue t i r e fa i lures d u r i n g a i r c r a f t ground operations. landing gear dynamic problems as wheel shimmy, truck pitching, and gear walk. Eventually these models could provide the information necessary to t a i l o r the mechanical properties of a i r c r a f t t i r e s t o make them more compatible with a i r c ra f t antiskid braking and nose gear steering systems and, thereby, optimize the ground handling capabili ty of modern a i r c r a f t .

A mature family of

The family of t i r e models should provide the means of solving such

CONCLUDING REMARKS

Langley Research Center i s conducting b o t h experimental and analytical t i r e research programs t o address the various needs identified by the landing gear designers and the airplane users. t o increase t i r e tread l ifetimes, re la te s t a t i c and dynamic t i r e properties, establish the t i r e hydroplaning s p i n - u p speed, study gear response t o t i r e fa i lures , and define t i r e temperature profiles during tax i , braking and cor- nering operations. The analytical programs are aimed a t providing insights into the mechanisms of heat generation in rol l ing t i r e s and developing the tools necessary to streamline the t i r e design process and t o aid i n the analysis of landing gear problems.

The experimental programs a re designed

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REF E R E N CE S

1. McCarty, John Locke: Wear and Related Character is t ics o f an Aircraf t Tire

2.

D u r i n g Braking. NASA TN D-6963, 1972.

Yager, Thomas J . ; McCarty, John L . ; R icc i t i e l lo , S. R . ; and Golub, M . A . : Development i n New Aircraf t Tire Tread Materials. Operating Problems, NASA SP-416, 1976, pp. 247-256.

Ai rcraf t Safety and

3. tlcCarty, John L.; Yager, Thomas J. ; and R icc i t i e l lo , S . R.: Wear, Friction, and Temperature Character is t ics of an Aircraf t Tire Undergoing Braking and Cornering. NASA TP-1569, 1979.

4. Horne, Walter B.; and Dreher, Robert C. : Phenomena of Pneumatic Tire Hydroplaning. NASA TN D-2056, 1973.

5. Horne, Walter B.; and Leland, Trafford J . W . : Influence of Tire Tread Pattern and Runway Surface Condition on Braking Friction and Rolling Resistance of a Modern Aircraf t Tire. NASA TN D-1376, 1962.

6. Smiley, Robert F . ; and Horne, Walter B. : Mechanical Properties of Pneumatic Tires w i t h Special Reference t o Modern Aircraf t Tires. NASA TR R-64, 1960.

7 . Tanner, John A . : Fore-and-Aft Elas t ic Response Characterist ics of 34 x 9.9, Type VII, 14 Ply-Rating Aircraf t Tires of Bias-ply, Bias- Bel ted , and Radial -Bel ted Design.

8. Sleeper, Robert K . ; and Dreher, Robert C . : Tire S t i f fness and Dampinq Determined From S t a t i c and Free-Vibration Tests. NASA TP-1671, 1980.

NASA TN D-7449 , 1974.

9. Saunders, S. L.: Nonlinear Theories fo r Thin Shells. Q. Appl. Math.; V O I . 21 , NO. 1 , 1963, pp. 21-36.

10. Budiansky, B. : Notes on Nonlinear Shell Theory. J . Appl. Ilech., Vol. 35,

11. Noor, Ahmed K. : Recent Advances in Reduction Methods f o r Nonlinear Problems. Computers arid Structures , Vol. 13, 1981 , pp. 31-44.

1968, pp. 392-401.

12. Noor, Ahmed K.; and Andersen, C. Pl.: Computerized Symbolic Manipulation in Nonlinear F in i te Element Analysis. Computers and Structures, VOI. 13, 1981, pp. 379-403.

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Figure 1.- Instrumented test vehicle.

WEAR FRICTION - ELASTOMER (SEE TEXT)

RATE,

0 0.5

SLIP RATIO

YAW ANGLE, DEG

BRAKING F/- FRICTION COEFF

0 0.5

SLIP RATIO

CORNERING

FRICTION COEFF o*8[K 25 0

YAW ANGLE, DEG

TEMPERATURE

SLIP RATIO

0.396

o c 0 2.50

DISTANCE. k n YAW, DEG

Figure 2.- Wear, friction, and temperature characteristics of a size 22 x 5.5 tire on a smooth asphalt surface.

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*: DATA FROM THESE THERMOCOUPLES COMPARED WITH MODEL PREDICTIONS

Figure 3 . - Location of thermocouples i n t i re carcass.

Figure 4.- Size 22 x 5.5 t e s t t i r e with thermocouples ins ta l led .

5 51

DISTANCE, rn 1500 3000 4500

TEMP, OC

)-----I 40 - 50 80 - 90 50 - 60 Esl 90 - 100 60 - 70 100 - 120 70 - 80

Figure 5.- Temperature p ro f i l e s during f r e e r o l l . 22 x 5.5, 12-ply t i re ; ground speed, 1 7 knots.

25% DEFLECTION 35% DEFLECTION

EXPERIMENTAL CALCULATED

_------ - e-c

INNER SURFACE

0 50 100 150 0 50 100 150 TIME, sec TIME. sec

OUTER SURFACE

Figure 6.- Comparison of experimental and calculated temperature rise. 22 x 5.5 t i re ; ground speed, 1 7 knots.

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- x I O 2 /--- -------

/’ - ------ --_-_____ ------

c /’

- STATIC RELAXATION

FREE VIBRATION

STATIC LOADING -

I I I 0 60 120 180

VERTICAL LOAD,kN

Figure 7.- Comparison of s ta t ic and dynamic lateral t i re spr ing rates €or a 49 x 1 7 tire.

INFLATION * 1

PRESSURE, MPa

MODEL CROSS SECTION U NI N FLATED

INFLATED

0 0.5 1 .o

CROWN DISP,crn

Figure 8.- T y p i c a l model r e s u l t s . 10-ply e l l ip t ica l t i re on a r i g i d wheel.

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